Capillary electrophoresis is a separation method employed in analytical chemistry which utilizes the differences in electrophoretic mobility of the sample substances to be separated. Capillary electrophoresis is used, for example, for separating different biological molecules, such as proteins or peptides. The separation process is performed in a capillary tube which is open on both ends and to which an electric field is applied which causes electrophoretic separation of different sample substances within the tube. The electric field is applied by means of electrodes which are arranged at the ends of the capillary, respectively, and which are connected to a high voltage power supply. The capillary is filled with an electrically conductive electrolyte so that an electric field can build up within the capillary. The two ends of the capillary are immersed in vials containing the electrolyte, respectively.
When new sample substances are to be introduced into the capillary for subsequent separation, the vial containing the electrolyte is removed from one end of the capillary, a vial containing the sample is positioned at this place so that the end of the capillary is immersed in the sample liquid. Thereafter, the sample is injected into the capillary by a suitable method, for example by applying a pressure above atmospheric pressure at the end of the capillary where the vial is positioned or a vacuum at the other end of the capillary. When the sample substances have been injected into the capillary, the sample vial is removed and the electrolyte vial is again positioned at this place. Thereafter, high voltage is applied so that electrophoretic separation of the sample substances takes place. At the end of the capillary opposite to the end of sample injection, a detector is arranged for detecting the separated sample substances by a suitable detection method, for example by a light absorption or a fluorescence technique.
After several electrophoretic separations have been performed, it may be necessary to draw off electrolyte from the electrolyte vial to avoid accumulation of products generated by electrochemical processes due to the current flowing in the capillary. If electrolyte has been drawn off or is otherwise missing in the electrophoretic system, it has to be replenished.
From the above, it becomes apparent that for performing capillary electrophoresis, a plurality of manipulations involving liquids have to be performed, such as immersing the two ends of the capillary in electrolyte, conveying sample liquid to an end of the capillary, replenishing the electrolyte. As these liquids are typically stored in vials, it is necessary to perform a plurality of movements of the vials and/or the separation capillary. In order to provide an electrophoresis device which is capable of automatically performing chemical analyses, an apparatus for handling liquid vials is required which performs the mentioned mechanical movements of vials and/or the capillary.
In the prior art, several different concepts for handling liquid vials in capillary electrophoresis are known. According to a first approach, a stationary electrolyte reservoir which cannot be changed is arranged at the outlet end of the capillary. The inlet end of the capillary is moved by a corresponding mechanism to the sample and electrolyte vials. This approach has several disadvantages: Since the ends of the capillary are not at the same level during the separation process, a hydrodynamic pressure difference is created between the ends of the capillary, whereby the separation efficiency is reduced. Furthermore, liquids for the preparation and for carrying out the electrophoretic separation can only be changed at the inlet end of the capillary. This limits the versatility of the apparatus. An additional disadvantage is that separated sample fractions cannot be collected at the outlet end of the capillary.
According to a second known solution for handling liquid vials, a stationary electrolyte reservoir is arranged at the outlet end of the capillary and a circular magazine containing sample and electrolyte vials is arranged at the inlet end of the capillary. A lifting mechanism is provided to lift sample and electrolyte vials such that the capillary end is immersed in the corresponding liquid. The disadvantages of this apparatus are like in the previously mentioned prior an apparatus that liquids for the preparation and for carrying out the electrophoretic separation can only be changed at the inlet end of the capillary. Furthermore, separated sample fractions cannot be collected at the outlet end of the capillary.
In another vial handling apparatus of the prior art, both ends of the capillary are fixed. Near both ends of the capillary, a circular magazine containing sample and electrolyte vials is provided. These two magazines may be operated independently of each other. The sample and electrolyte vials may be lifted so that the capillary ends dip into the corresponding liquid. A liquid handling apparatus of this type is known from EP-A-0 339 779. This known apparatus comprises two concentric conveyors which can be rotated around a common axis. The conveyors comprise septum sealed vials which can be conveyed into registry underneath a cartridge containing a capillary for capillary electrophoresis. The vials, each held in a vial holder are conveyed until one vial on one conveyor underlies one depending capillary end and the other vial on the other conveyor underlies the other depending capillary end. When registry of the vials to the capillary ends has been made, the vials are moved upwardly by piston-assisted movement of vial holders with respect to the-conveyors until the vials are pierced at a sealing septum by hypodermics. After piercing by the hypodermics, the piston-assisted movement continues to thread the hypodermic with either a capillary or an electrode for access to the content of the interior of the vial.
The known apparatus has the disadvantage that it is mechanically very complex since it requires two magazines for the vials, each with a driving motor and a lifting mechanism. Furthermore, there is a fixed correspondence of the vials to a certain end of the capillary, i.e., the vials which can be positioned at the inlet end of the capillary cannot be positioned at the outlet end of the capillary and vice versa.